Adjustable coaxial line circuit element



Ap 1953 R. G. MIDDLETON ADJUSTABLE COAXIAL LINE CIRCUIT ELEMENT Filed March 24, 1950 INVENTOR ATTORNEY Patented Apr. 14, 1953 ADJUSTABLE COAX'IAL LINE CIRCUIT ELEMENT Robert G. Middleton, Woodside, N. Y., assignor to Radio Corporation of America, a corporation of Delaware Application March 24, 1950, Serial No. 151,712

11 Claims.

This invention relates to coaxial line circuit elements and more particularly to continuously variable resonant line elements employing coaxial transmission lines.

In the field of microwave equipment, it is desired to have available resonant line sections of various and variable lengths. These sections are not only useful in research and development laboratories, but also are desired and required in the assembly and use of microwave equipment wherein they are employed as resonant or reactive circuit elements.

Heretofore, coaxial cables have had. to be cut separately to the desired lengths. Their use was limited to their cut lengths and there was no convenient way, heretofore, to vary them in effective length.

The principa1 object of the invention is to provide a continuously variable resonant line section that is highly efiicient, is low in cost, is compact and easy in adjustment.

Another object is to provide a coaxial line circuit element having an effective length which is continuously adjustable.

Another object of the invention is to provide such a resonant-line that is tunable in effective length by a rotary control.

Other objects will be apparent from the description of the invention as hereinafter set forth in detail and from the drawing made a part hereof in which:

Fig. 1 is a cross-sectional view of a coaxial cable in which a polystyrene string is wound around the inner conductor to maintain the desired coaxial conductor positions;

Fig, 2 is a side elevational view of one embodiment of the invention with a portion cut away to show the interior details of the device and wherein the coaxial cable of Fig. 1 is wound into a cylindrical helix;

Fig. 3 is a side elevational view of another embodiment of the invention with a portion cut away to show the interior details of the device and wherein the coaxial cable of Fig. 1 is wound into a flat spiral helix; and

Fig. 4 is a cross-sectional view taken on line 4--4 in Fig. 2.

Similar reference character are applied to similar elements throughout the drawing.

Referring to Fig. 1, the outer conductor I of a coaxial cable surrounds the inner conductor 2 thereof. A polystyrene string wound in a spiral inside the outer conductor I and around the inner conductor 2 maintains the conductors in the desired coaxial relation to form a coaxial line with a partial air dielectric.

In the embodiment of the invention shown in Fig. 2 a suitable length of coaxial line preferably of the type shown in Fig, 1 is formed into a cylindrical helix. This helix is supported by a r0- tatable shaft in any suitable conventional manner. One end of shaft 4 is supported in bearing 5 and the other end of shaft 4 is supported in a bearing consisting of a disc 6 of insulating material with two slip rings 1 and 8 for contact respectively with brushes 9 and I0, which are connected to the line inner and outer conductors l 2 respectively. A calibrated knob H is keyed to the shaft 4. If only limited adjustment is required, flexible connections to the line conductors may be substituted for the slip rings and brushes. A quantity of mercury 12, sufficient in volume to short-circuit the line conductors for a small portion of their lengths, is disposed within the helix. A suitable volume is indicated in Figs. 2 and 4 by line l3l3 which intercepts the inside walls of the outer line conductor I above the level of the upper wall thereof at the bottom section of an individual convolution of the helix.

In the embodiment of the invention in Fig. 3, the line conductors are formed in a flat spiral helix (in the plane of the drawing) and the spiral helix is rotatable by shaft 4. Slip rings or flexible connections may be used to connect to the spiral conductors as shown and described heretofore with respect to the embodiment of Fig. 2.

The cylindrical ends of the helical or spiral line sections preferably are sealed to prevent evaporation of the mercury and to prevent the loss of the mercury should the helix or spiral be tilted or rotated too far on its axis.

The line conductors should consist of, or be plated with, some metal, such as nickel, with which the mercury will not amalgamate.

In operation, when a suitable amount of mercury is disposed in the lower portion of a convolution of the helix, as described herein, the effective length of the coaxial line extends from the end at which the connections are made to the point in the helix where the line is short-circuited. Thus the effective length of the line can be varied along the helix by rotating the shaft 4, since the mercury, being a liquid, will remain at the bottom of an individual turn of the helix. The helix may be rotated through as many degrees of angle as desired to provide the required tuning adjustment.

If the device described are operated at sufficiently high frequencies, well known difficulties may be encountered with the use of the brush-slip ring or flexible lead contact arrangements disclosed. These objections can be overcome by substltuting a conventional jointed rotatable coaxial line for the actuating shaft whereby effectively rigid connections may be made between the helix and the utilization circuits.

What is claimed is:

1. A continuously variable resonant-line section comprising: a coaxial transmission line in the form of a helix, a liquid short-circuiting material in an amount suiiicient to short-circuit the line in one convolution of said helix, and means for rotating said helix about its axis.

2. A continuously variable resonant-line section comprising: a coaxial transmission line in the form of a helix, 3, liquid short-circuiting material in an amount to short-circuit the line in one convolution of said helix, means for rotating said helix about its axis, and means for connecting said section to external circuits.

3. A continuously variable resonant-line section comprising: a coaxial transmission line in the form of a helix, mercury in one convolution of said helix of suflicient volume to short-circuit the conductors of said line, and means for rotating said helix about its axis.

4. A continuously variable resonant-line section comprising: a coaxial transmission line in the form of a helix, means for rotating said helix about its axis, and means for progressively shortcircuiting the line conductors of said helix along said line in proportion to the extent of rotation of said helix.

5. A continuously variable resonant-line section comprising: a coaxial line in the form of a helix, means for rotating said helix about its axis, and means for progressively short-clrcuiting the line conductors of said helix along said line in linear proportion to the extent of rotation of said helix.

6. A continuously variable resonant-line section comprising: a coaxial line in the form of a helix, means for rotating said helix about its axis, and means for progressively short-circuiting the line conductors of said helix along said line in geometric proportion to the extent of rotation of said helix.

7. A continuouslyvariable resonant-line section comprising: a coaxial line in the form of a helix-means for closing the ends of said helix, mercury in one turn of said helix of sufiicient volume to short-circuit the conductors of said line for at least a'portion of said turn, and means for rotating said helix about its axis.

8. A continuously variable resonant-line section comprising: a coaxial transmission line including inner and outer conductors, the said conductors being in the form of a helix, mercury in one turn of said helix of suificient volume to short-circuit said conductors for at least a portion of said turn, means for rotating said helix about its axis, and means for minimizing the reaction of said mercury and the inner walls of said tube.

9. A continuously variable resonant-line section comprising: a coaxial transmission line in the form of a cylindrical helix, a shaft for supporting said helix on its axis, mercury in one turn of said helix of sufficient volume to shortcircuit the conductors of said line for at least a portion of said turn, means for rotating said shaft about its axis, and means for connecting said section to exterior circuits.

10. A continuously variable resonant-line section comprising: a coaxial transmission line in the form of a spiral helix, a shaft for supporting said spiral helix on its axis, mercury in one turn of said spiral helix of suificient volume to shortcircuit the conductors of said line for at least a portion of said turn, means for rotating said shaft about its axis, and means for connecting said section to exterior circuits.

11. A transmission line of continuously variable length comprising a transmission line section having a metallic pipe for guiding therewithin radio waves and in the form of a helix, a liquid metallic short-circuiting material in one convolution of said helix in a quantity to obstruct said waves, and means for rotating said helix about its axis.

ROBERT G. MIDDLETON.

References Cited in the file of this patent UNITED STATES PATENTS Number Name Date 1,641,491 Hyland Sept. 6, 1927 2,121,855 Buschbeck June 28, 1938 2,402,739 Doughty June 25, 1946 2,427,098 Keizer Sept. 9, 1947 OTHER REFERENCES Websters New International Dictionary, second edition, unabridged, 1947, G. & C. Merriam Co., Springfield, Mass, page 141, col. 1. 

